A dye-sensitized solar cell has been attracting attention as a new solar cell substituting a silicon-based solar cell since a photoelectric conversion element comprising dye-sensitized semiconductor fine particles was proposed (Nature, vol. 353, p. 737 to 740 (1991)).
A dye-sensitized solar cell comprising a plastic substrate attracts attention because it can be made soft and lightweight. In the case of a dye-sensitized solar cell comprising a glass substrate which is commonly used, a high-temperature heat treatment is carried out to form a porous structure so as to enhance integrity among oxide semiconductor particles and improve photoelectric conversion efficiency. However, the temperature is generally 400° C. or higher, and it is difficult to carry out a high-temperature heat treatment directly on the plastic substrate. To cope with this, in JP-A 11-288745, a dye-sensitized solar cell comprising a plastic substrate is manufactured by oxidizing metal foil and making the surface of the metal foil uneven. However, the specific surface area of the dye-sensitized solar cell is not sufficiently large, whereby photoelectric conversion efficiency is not fully improved. In JP-A 2001-160426, after the high-temperature heat treatment of a metal oxide is carried out on metal foil, the metal oxide layer is removed and fixed on the plastic substrate by a binder. However, this process is complicated and not suitable for mass-production. In JP-2002-50413, metal oxide particles are coated on a plastic substrate to form a semiconductor metal oxide layer. However, the metal oxide particles fixed on a transparent conductive layer fall off in a powdery form at the time of handling or peel off in an electrolyte.
JP-A 2001-93590 and JP-A 2001-358348 disclose that a metal oxide needle-like crystal is used as an electrode for solar cells to improve charge transport efficiency. However, to attain high charge transport efficiency by obtaining a good porous structure, the crystal state of the metal oxide must be properly controlled. For example, in the case of titanium oxide as the metal oxide, an anatase phase is preferred. However, it is difficult to manufacture needle-like titanium oxide having an anatase phase, and titanium oxide having a more stable rutile phase is first formed. As a result, photoelectric conversion efficiency is not satisfactory.
Meanwhile, electrospinning is one of the methods of manufacturing a metal oxide. In this method, an oxide precursor containing a burned-out component such as a polymer is ejected onto a substrate at a high aspect ratio and heated at a high temperature to obtain a metal oxide. An electrode for dye-sensitized solar cells having a metal oxide layer on a glass substrate by using this electrospinning has already been known. The above dye-sensitized solar cell is described in US2005/0109385 and “Nanotechnology” written by Mi Yeon Songs et al., p. 1861 to 1865, 2004.
In the above-described electrode for dye-sensitized solar cells, a metal oxide precursor is ejected onto a transparent conductive layer overlying a glass substrate at a high aspect ratio to be deposited and baked at a high temperature to obtain a metal oxide layer. The metal oxide tends to peel off from the transparent conductive layer due to the shrinkage of the metal oxide at the time of baking. Even when the metal oxide layer is formed by electrospinning, a sufficiently large specific surface area and sufficiently high charge transport efficiency cannot be obtained. Since the step of baking the metal oxide over the glass substrate is carried out at 400° C. or higher, it is difficult to apply this technology to an electrode for dye-sensitized solar cells which comprises a plastic substrate.